Clinical and morphologic observations are described in 12 patients who underwent simultaneous replacement of the tricuspid, mitral and aortic valves. All 12 patients had mitral stenosis; 10, aortic valve stenosis and 2, pure aortic valve regurgitation; 5 had tricuspid valve stenosis and 7, pure tricuspid valve regurgitation. Of the 10 patients dying within 60 days of triple valve replacement, 7 had the low cardiac output syndrome which in 4, and possibly 5, of the 7 was attributed to prosthetic aortic valve stenosis. In none of the 12 patients was the ascending aorta dilated, and in these 4 (possibly 5) patients with the low cardiac thought, the space between the surface of the caged poppet (4 patients) or margins of the tilting -disc (1 patient) in the aortic valve position and aortic endothelium appeared inadequate to allow unobstructed flow despite small-sized prostheses in all but 1 patient. Thus, aortic valve replacement in the setting of triple valve dysfunction is hazardous or potentially so. The relative small sizes of the hearts in these patients also makes valve replacement more difficult (and hazardous) compared to hearts with larger sized ventricles and aortas.